The present invention relates to an exhaust gas purification technology for removing particulate matters from exhaust gas discharged from an internal combustion engine.
It is demanded of an internal combustion engine installed in a motor vehicle or the like to improve exhaust emission properties by purifying noxious gas contents contained in exhaust gas such as nitrogen oxides (NOx) and hydrocarbon (HC). Especially in the case of a diesel engine, it is also important to reduce so-called particulate matters (PM) such as soot and SOF (soluble organic fractions) as well as nitrogen oxides (NOx) and hydrocarbon (HC).
To meet such a demand, xe2x80x9can exhaust gas purifier for an internal combustion enginexe2x80x9d as disclosed in Japanese Patent Gazette No. 2722987 has been proposed.
The exhaust gas purifier for the internal combustion engine disclosed in the above-mentioned publication is constructed as follows. A particulate filter for trapping particulate matters in exhaust gas is provided in an exhaust passage of the internal combustion engine. An NOx absorbent, which absorbs NOx in exhaust gas when the exhaust gas demonstrates a lean air-fuel ratio and which discharges and reduces the absorbed NOx when the exhaust gas demonstrates a low concentration of oxygen and there is a reducing agent, is disposed at such a location that heat transfer to the particulate filter is possible. The exhaust gas purifier for the internal combustion engine thus constructed is intended to burn and remove particulate matters trapped in the particulate filter by raising a temperature of the particulate filter with the aid of heat generated at the time of an oxidizing reaction of the reducing agent on the NOx absorbent.
By the way, agglomeration of compounds called ashes may clog exhaust flow passages in the particulate filter and increase pressure loss in exhaust gas.
The mechanism of production of ashes has not been unriddled yet but is hypothesized as follows.
That is, fuel or lubricating oil for the internal combustion engine (so-called engine oil) contains various additives and impurities. These contents bond to one another in combustion chambers of the internal combustion engine or on the particulate filter, thus forming various compounds. These compounds agglomerate on the particulate filter and form ashes.
For example, fuel or lubricating oil for the internal combustion engine contains sulfur (S) contents, phosphorus (P) contents, calcium (Ca), magnesium (Mg) and the like. The contents contained in blow-by gas (lubricating oil) bond to the contents contained in mixture (fuel), whereby compounds such as calcium sulfate (CaSO4), calcium phosphate (Ca3(SO4)2) and magnesium sulfate (MgSO4) are produced. The calcium sulfate (CaSO4), calcium phosphate (Ca3(SO4)2) and magnesium sulfate (MgSO4) are trapped onto the particulate filter together with particulate matters (PM) and are agglomerated as ashes.
Because sulfur (S) has a property of high absorbency into soot, the sulfur (S) that has been absorbed onto the particulate filter together with soot bonds to calcium (Ca) and magnesium (Mg) in exhaust gas and thus produces compounds such as calcium sulfate (CaSO4) and magnesium sulfate (MgSO4). These compounds are agglomerated as ashes.
If ashes are produced in the particulate filter in this manner, the exhaust gas in the particulate filter is clogged to such an extent that pressure loss in exhaust gas increases. Therefore, it is important to suppress the production of ashes in the particulate filter.
The present invention has been made in view of the aforementioned circumstances. In an exhaust gas purifier having a trap for trapping particulate matters contained in exhaust gas discharged from an internal combustion engine, the present invention aims at preventing pressure loss in exhaust gas associated with the clogging of the trap by providing a technology which makes it possible to suppress the production of ashes.
The present invention has adopted the following measures in order to achieve the above-mentioned object. That is, the present invention provides an exhaust gas purifier for an internal combustion engine having a trap for trapping particulate matters contained in exhaust gas discharged from the internal combustion engine, and wherein the trap is constructed to carry a metal which has an electronegativity equal to or lower than a predetermined content contained in fuel or lubricating oil for the internal combustion engine.
Also, according to the present invention, an exhaust gas purifying method for an internal combustion engine comprising the steps of: carrying a metal which has an electronegativity equal to or lower than a predetermined content contained in fuel or lubricating oil for the internal combustion engine in a trap for trapping particulate matters contained in exhaust gas discharged from the internal combustion engine; and suppressing condensation of a sulfur content discharged from the internal combustion engine and the predetermined content on the trap.
In the exhaust gas purifier thus constructed, the trap traps the particulate matters contained in exhaust gas discharged from the internal combustion engine.
In this process, the exhaust gas contains contents which are originally contained in fuel and/or lubricating oil for the internal combustion engine. A predetermined one of those contents may bond to other contents (hereinafter referred to as bonded contents) on the trap and may produce ashes.
However, the trap carries a metal which has an electronegativity equal to or lower than the predetermined content, preferably a metal which has an electronegativity lower than the predetermined content and which has an ionization tendency greater than the predetermined content. In other words, the trap carries a metal which bonds to the bonded contents with a greater bonding strength than the predetermined content.
Thus, the bonded contents bond to the metal carried on the trap instead of bonding to the predetermined content. As a result, production of ashes in the trap is suppressed.
Compounds of the metal carried on the trap and the bonded contents may be agglomerated on the trap as is the case with ashes. It is preferable to select the metal such that those compounds are decomposed or purified under a condition similar to the condition for purification of particulate matters.
Thus, the exhaust gas purifier for the internal combustion engine in accordance with the present invention makes it possible to suppress formation of ashes in the trap, prevent the clogging of the trap associated with ashes, and obviate pressure loss in exhaust gas.